<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>网页开关灯</title>
    <script type="text/javascript">
        window.onload = function() {
            var btn = document.getElementById('btn')
            var flag = 0
            btn.onclick = function() {
                if (flag == 0) {
                    document.body.style.backgroundColor = 'black'
                    flag = 1
                } else {
                    document.body.style.backgroundColor = '#fff'
                    flag = 0
                }
            }
        }
    </script>
</head>

<body>
    <button id="btn" onclick="">开关灯</button>
</body>

</html>